In general, a bulldozer blade is raised and lowered by a double acting hydraulic actuator. A work machine such as a bulldozer includes a hydraulic system for driving the hydraulic actuator. An oil tank and a hydraulic pump, which configure the hydraulic system, are connected to the hydraulic actuator via conduits. Also, a direction control valve is connected to the conduits. The direction control valve controls the moving direction of the blade, which is selectively raised and lowered. The direction control valve is switched by a manipulation lever between a position for raising the blade from a neutral position and a position for lowering the blade from the neutral position. When leveling a ground by the bulldozer, in order to increase operation efficiency, an operator rotates the manipulation lever for raising and lowering the blade to the maximum operation angle to quickly drop the raised blade using its own weight.
Since the blade is heavy, quickly dropping the blade creates negative pressure inside actuator chambers and pipes. When the inside of the actuator chambers and the pipes is under negative pressure, air bubbles are generated in pressurized oil, and the operation of the hydraulic system might cause failure. To eliminate the problem, a quick drop circuit disclosed in patent document 1 is provided with a quick drop valve located in conduits between a direction control valve and hydraulic actuators. The hydraulic actuators are hydraulic cylinders each provided with a piston, and each includes first and second actuator chambers located on both sides of the piston. The blade is raised when oil is supplied to the first actuator chambers of the hydraulic actuators, and lowered when oil is supplied to the second actuator chambers of the hydraulic actuators. When quickly dropping the load, compressed high pressure oil in the first actuator chambers is directly supplied to the expanded second actuator chambers via the quick drop valve. Accordingly, the blade is quickly dropped while preventing generation of negative pressure in the actuator chambers and the pipes.
Patent document 1: Japanese Laid-Open Patent Publication No. 7-167107